A Compact High-Torque Twisted String Actuator Using Parallel Bundle-Driven Actuation With an Asymmetric Behavior Compensator

The twisted string actuator (TSA) has been extensively studied as an alternative to metallic gear reducers due to its low-cost, lightweight, compliance, and highly adjustable actuation characteristics. Owing to these advantages, TSAs are utilized to realize lightweight human–robot interactive robots requiring safe interaction, such as wearables and prosthetics, at a relatively low cost. However, because its force transmission ratio is almost proportional to the length of the string, the string of the conventional TSA must be elongated to achieve high force output. Thus, it is difficult to obtain high force capacity while maintaining a small size. To overcome this limitation, in this article, we propose a novel TSA based on parallel bundle-driven actuation to significantly reduce the length of the TSA while achieving high force output. Furthermore, in order to boost the proposed TSA performance, we also present a passive mechanism of an asymmetric behavior compensator. The experimental results show that the force transmission ratio of the proposed TSA is increased by 21%, compared with that of the conventional TSA. Consequently, the proposed TSA has a 44% higher torque capacity with a 37% shorter string than those of theconventional TSA.